config/toolchain: force use of sysroot if OBSOLETE is not set

Use of the sysroot is highly recommended, and the non-sysroot case is
both obsolete and not widely tested.

Before the non-sysroot case can go away, deprecate it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =